Examples of known curable compositions which contain mercapto group-containing polymers as essential components and which can be cured by disulfide bond formation reaction include compositions containing a polysulfide polymer, such as Thiokol (manufactured by Toray Thiokol Co., Ltd.); a modified polysulfide polymer, such as Permapol (manufactured by Nippon Shokubai Co., Ltd.); a mercapto group-terminated polyisobutylene-based polymer disclosed in Japanese Unexamined Patent Application Publication No. 2001-106726; or the like as an essential component. These compositions are used for sealants, oil-resistant coating materials, lining materials, hose materials, printing rollers, cold casting materials, potting materials, modifiers for paint and adhesives, softeners, etc.
However, in some cases, polysulfide polymers may discolor or soften finishing agents or paint applied to surfaces of cured objects, which gives rise to problems particularly when used as elastic sealants. With respect to the compositions using the polyisobutylene-based polymer disclosed in Japanese Unexamined Patent Application Publication No. 2001-106726, cured objects exhibit poor heat resistance, oil resistance, and weatherability. The modified polysulfide polymer has a long curing time in the inner regions. Furthermore, in polysulfide polymers and modified polysulfide polymers, it is difficult to accurately control the molecular weight or narrow the molecular weight distribution, and hence it is difficult to accurately control the physical properties, such as moduli and elongation, of cured objects, which are affected by the molecular weights and molecular weight distributions.